The Sportster and Buell Motorcycle Forum - The XLFORUM®

The Sportster and Buell Motorcycle Forum - The XLFORUM® (
-   Sportster Motorcycle - Bottom End (
-   -   Sportster Crankcase Pressure / Engine Breathing / Wetsumping and Mods (

Hippysmack 1st July 2019 21:54

Sportster Crankcase Pressure / Engine Breathing / Wetsumping and Mods
I have created a page in REF section of the Sportsterpedia for Evo Crankcase Pressure.
That will eventually spin off an IH Crankcase Pressure article.

There are many threads on crankcase pressure and related topics spread all over the forum.
The Sportsterpedia article is a compilation of this forum (plus what I've learned along the way).
And in so being, each format gives to the other.
Information from the forum went into creating the pedia article.
Likewise, I'm bring that article into this thread for further exposure and discussion.
Hopefully it'll be helpful to all of us.

I'm no expert, but I am trying to learn as much as I can about Sportster engine breathing.
Key phrase "Sportster Engine Breathing".
Different year models have their differences.
But engine breathing / crankcase pressure / Wetsumping and the like are conditions we all have in common, with exceptions of course.
So this thread is dedicated to ALL Sportster engines.
Input from the IH and Evo community from 57 to present would be greatly appreciated.
Novice to experts welcomed.
This is not meant as a tutoral but more of a discussion.
I've probably got more to learn than most actually.
But let's see where we can go with it.

Dr Dick once called this subject one of the "Dark Arts" of Sportsterdom.
I understand why completely.
There are forces inside a running engine that are real difficult to understand.
As bustert once alluded to, what you can prove on paper still doesn't match what is going on inside.
So this article is created based on facts as well as theory.
As well as some science (most of which is over my head).

Related articles in the Sportsterpedia:
(all of these pages can be accessed by the main Tech Menu:

IH Crankcase Ventilation:
IH Engine Oiling:
IH Oil Pump Main Page:
Evo Crankcase Ventilation:
Evo Engine Oiling:
Evo Oil Pump Main Page:
Evo Crankcase Pressure:
Wet Sumping:
Engine and Primary Oil System Modifications:
Cylinder Compression Test:
Cylinder Leak-Down Test:

Breathers and Testing Videos:

DK Custom Breather System Air Volume Testing of Harley-Davidsons
Is your Harley making a mess? MFAQ about Oil & your Motorcycle (from DK Custom)
FAQ's External Breather Systems for Harley by DK Custom

Hippysmack 1st July 2019 21:56

--Crankcase Pressure --
There are many different XLF member's comments in this article.
Credits have been omitted here to keep this as a discussion.
However, all credits are in the Sportsterpedia article.

I did eventually list the credits but they were eventually taken out. :dunno
So go to the Evo Crankcase Pressure and Engine Breathing article in the Sportsterpedia and look at the very bottom of the page for the credits. :shhhh

In the crankcase air system;
Crankcase air pressure is mainly generated by the up and down movement of the pistons.
Additional air pressure is created by blowby from the combustion chambers past the rings and into the crankcase.

It helps to push sump oil up and out the scavenge passage to the return side of the oil pump.
(the scavenge side of the pump also pulls a vacuum on the sealed passage from the sump outlet to the pump)
It also initiates the splash and mist process as the compressed air above the oil is ready to spring up when the piston rises.

* Using a 1000cc motor:
Consider the pistons as they stroke up and down in the cylinders.
It's the down action that draws fresh intake charge into the cylinders, where it burns propelling you down the road.
Then the pistons expel the waste as exhaust when they travel up.
We all know this. 500cc is what each piston 'sweeps'.
This all happens on the topside of the pistons.

That same 500 each (1000 total) is also getting displaced on the underside of the pistons, compressing it into the crankcase.
Instead of into the cylinder head.

Intake and exhaust valves in the cases:

The piston up and down movement in the cases get pressurized and then 'relaxed'.
(like if you blow into a balloon it pressurizes then breathe in and it relaxes)
If you take the timing plug out of a running engine, this opens the crankcase to the atmosphere.
Then you'll get 1000cc of air/oil mixture blowing out of it followed by 1000cc getting sucked back in.
On 77 and up motors, there are holes through thru the wall separating the crank case from the cam case.
These holes are the same as 'opening the timing plug'.

It's the blowing out oil-air that we should address 1st.
The oil that's getting blown out is oil that got pumped to the big end bearings through the pinion shaft.
(and from the piston squirters on 04 and up models)
The oil did it's thing for the crankpin bearings (and piston lowers) and then flowed out into the case where it mixes with the air in case.

Mixing isn't really the right term.
Suspended is.

The oil is suspended in the air just like rain water is suspended in air in the wake of a tractor trailer in the rain.
The faster the tractor trailer drives, the better the suspension.
Or, the higher the engine rpm the better the oil suspension.

The drain down oil from the top end (and oil squirters respectively) is also introduced into the case air but its only a fraction of what came from the crankpin.
(and piston squirters respectively)

This used oil needs to be evacuated (to the oil tank) so that the case doesn't fill up with oil.
The downstroke of the pistons causes the volume underneath the pistons to decrease which puts positive pressure in the lower end chambers.
This also puts positive pressure on the oil in the sump.

This pressure is multi use;
Then the upstroke of the piston creates an upward negative (vacuum) pressure bringing some of the oil from the sump with it (suspended).
With little to no (piston ring) blow-by and a check valve on the breather system;
Crankcase pressure is essentially cycling between (positive and negative pressures) as the pistons go down and back up.
(remember, due to the common crankpin 45 degree design, a Harley motor is a variable volume crankcase, unlike most motors)

This creates splash oil which is bounced about in the crankcase.
This also creates an air / oil mix when tiny particles intertwine with the oil in suspension.
The two don't actually mix as does sugar and water.
So separating them back apart is fairly easy if you add an obstacle for that 'mix' to collide into.
The obstacle is widely known as the breather or umbrella valve although anything the mix touches in the motor could accomplish the same thing in theory.

Instead of the oil-air getting blown into the garage, HD blows & sucks it into the cam chest where it accumulates on the walls of this smallish area.
Imagine the tractor trailer going into a tunnel.
That suspended rain water will collect on the walls of the tunnel and flow down into the storm drain.

Or, in our case, to the return side of oil pump and then back to oil tank.

The piston motions create a pulsating blast of air pressure (push pull condition as each piston rises and falls).
Static oil pump pressure has already been dissipated by the time it reaches the crankcase.
(although it takes static oil pressure to get the oil from the pump to the crankcase)

Likewise, crankcase (CC) pressure will have a constant change in velocity.
Oil in the crankcase adds resistance to the air pressure generated (raising the pressure).
The movement of the pistons and flywheels splash oil around in the engine.
Gravity oil (from the drain ports in the heads) returns to the crankcase or gearcase (respective to year model);

On 86-03 engines, gravity oil falls into the crankcase sump area.
On 04 and up engines, gravity oil falls into the gearcase.

CC pressure both pushes to and sucks from the breather valve.
So testing full CC pressure with a PSI gauge would probably just destroy it.

The volume between positive and negative pressure decreases as RPM goes up.

Hippysmack 1st July 2019 21:57

--Affects of the 45° Rod / Piston Arrangement --
Piston / rod positions are relative to positive or negative pressure in the crankcase. Forget about valve opening for a second.

1. With the piston up,
The top of the piston is not pulling vacuum in the cylinder.
The crankcase is pulling vacuum.
2. With the piston down,
The piston is pulling vacuum in the cylinder.
The crankcase is exerting positive pressure.

These two conditions create what we call crankcase pressure.
How crankcase pressure moves inside the engine does or doesn't help during wet sumping.

Intake vacuum is relative to piston / rod positions.
On carbed models with VOES / MAP:
Vacuum is created in the cylinders which pulls vacuum from the carb.
This vacuum is tapped into and used to operate ignition timing through the VOES / Map.
So a vacuum gauge plugged into the vacuum line at the carb does reflect the position of the two pistons.
I.E. the gauge moves when the pistons are on the way down and goes toward the resting position when the pistons are coming up.
(of course, you'd then have to take in accountancy if the intake or exhaust valve was open at the time)

Some time and tune older V-8s with a vacuum gauge instead of a timing light.
For each piston going up, there is a piston going down.
Intake vacuum stays (more) constant in the middle of the push / pull.
A vacuum gauge shows a more steady dynamic condition.
Likewise are the forces in the crankcase (crankcase pressure).

The rods \ piston action in the Sportster engine are close together (45° apart).
There are 360° around the flywheels.
If the rods were 180° apart instead, one piston would be going up at the same time the other is going down.
Just as in a V-8, crankcase pressure would be more equalized between them.

Since Sportster piston movement is not equalized, we get the potato, potato sound we all love but the equilibrium in the crankcase is off by design.
Take the PCV off the valve cover on the V-8 while running and it just smokes a little.
Take the breather valve off the Sportster and oil pukes out.

Likewise, a vacuum gauge on the carb line will be erratic and not a useful test.
Hook up a vacuum gauge to it and the gauge bounces from vacuum to no vacuum pretty wildly.
Crankcase pressure is doing the same thing.

Hippysmack 1st July 2019 21:58

--Vacuum Pressure --
Vacuum and (positive) pressure are the terms that describe the amount of molecules of a gas in a given unit of space.
More molecules inside the engine than outside = inside air pressure.
Less molecules inside the engine than outside = inside is vacuum pressure.

Image two scenarios:
1. The case is sealed (closed to atmosphere).
The pistons just compress and relax the fluid in the case.
2. The case has a huge passage that allows the fluid to pass into and from the atmosphere.

Which takes more power to cycle?
Possibly the second.
Because the 1st, as the pistons use power to compress the fluid;
That power is returned as the pistons rise from the stored energy in the compressed fluid pushing pistons up.

That may be what's really happening for the most part in our bikes.
Obviously, the breather opening is too restrictive to allow so much flow that the case pressure stays more constant.
That restriction is on purpose to lessen the pumping losses by being closer to #1 than to #2.

As the vacuum increases, the pumping losses decrease and the fluid is less dense.
On the flip side, less dense fluid can't suspend as much liquid (possibly resulting in more liquid drag).

Below is some terminology for vacuum measurements.
PSIG - (pounds per square inch (gauge):
Gauge pressure is pressure measured relative to ambient atmospheric pressure (approximately 14.7 PSIA).
PSIA - pounds per square inch (absolute):
Absolute pressure is measured relative to high vacuum (0 PSIA).
PSIV - pounds per square inch (vacuum):
Vacuum pressure is measured relative to ambient atmospheric pressure.
PSID - pounds per square inch (differential):
Differential pressure is pressure measured relative to a reference pressure.
If the reference pressure is one atmosphere the differential pressure range is equal to gauge pressure range.

The earth's atmosphere exerts a pressure upon us, known as the atmospheric pressure, which can be measured in a number of ways. 13)
At sea level, the standard pressure is 14.7 psia or 29.92“ of mercury (Hg) or 760 mm of mercury (Torr).
Because the barometric pressure varies, the above “sea level” pressures are used as a reference point.
There is 14.7 psia pressure being exerted on us by the atmosphere, but there is also 14.7 psia inside of us pushing out.
(given the fact that for every action there is an equal but opposite reaction)
Thus, we do not feel discomfort from the atmospheric pressure.
Another way to state this is that there is no differential pressure between the inside and outside of our bodies.

The term “vacuum” is used to describe the zone of pressure below atmospheric pressure.
Vacuum is a negative gauge pressure, usually referenced to the existing standard barometric pressure where the equipment will operate.
This means vacuum is a differential reading between the surrounding atmospheric pressure and the pressure in the system evacuated.
In all instances when given a vacuum condition, the question should be asked, at what elevation the pump will operate.
(since the barometric pressure varies with altitude above or below sea level)

Example of differential pressures (or Vacuum):

Applying the same principles, you can see the relation of positive and negative crankcase pressure in the Sportster engine.
Every piston upstroke and downstroke reverses crankcase pressure from positive to negative forces.
The pistons act as an air pump and then a suction pump respectively.
Below are drawings exampling positive and negative (vacuum) forces inside the engine.

86-90 models with cam chest breather vents:

91-97 models with head breather vents:

98-03 models with head breather vents:

04 and Up models with head breather vents:

The breather valve is necessary to keep the imbalance in the crankcase (from the 45° arrangement) at bay.
The umbrella closes when the upstroke happens. It's not mechanical. Vacuum pulls it closed.
The forces are simple in that during upstroke, a negative pressure is pulled inside.
The outside air has a higher pressure than the inside air does.
So the outside air tries to enter the engine.
If the umbrella closes, it keeps outside air from entering.
If the umbrella stays open or doesn't seal fully, the higher pressure from atmosphere enters the crankcase.

In most engines negative crankcase pressure allows less ring pressure and the combination of both means more hp.
Over the years folks have used exhaust system energy to pull pressure from the case for this reason.
Guys have won championships with an engine that had an electric pump to reduce crankcase pressure.
Crankcase pressure in these engines fluctuate wildly from positive to negative.
At some point, it can have a dramatic affect on scavenging.

Oil scavenging:
Positive crankcase pressure aids scavenging.
Negative pressure makes the pump's job harder, because the pump is fighting the crankcase vacuum.
(with little to no blow-by and a check valve on the breather system)

Hippysmack 1st July 2019 22:08

--Symptoms of High Crankcase Pressure--
Picture a balloon inside the engine being blown up.
It puts internal pressure against the weakest structural points (gaskets and seals).

Symptoms include:
Sweating oil from the cylinder base gaskets and rocker boxes.
As well as the push rod tubes and lifter blocs on the other side.

Hippysmack 1st July 2019 22:11

Blowby pertains to the condition of ring seal at the cylinders / pistons.
Combustion above the piston is pushed past the rings and into the crankcase.
What actually 'blows by' the rings and into the crankcase is a mix of unburnt fuel, water, soot, acids etc.

Regarding blowby at high revs, the key factor is ‘ring-flutter’ or ‘ring-seal’.
Starting with a dead cold engine, there’s no blow-by measurable for a time after start-up.
Then at warm idle, it starts. Above idle it either drops off or holds constant, as revs go up.
Ring seal improves above idle.
As you get near peak revs, ring-seal again fails and gusts of blow-by pass down into the crankcase.

Combustion chamber blowby adds positive pressure to the crankcase at the same time it's being lowered by the vacuum condition on piston upstroke.
The greater the blowby, the more it pressurizes the crankcase during this period.
By design, we have a little blowby bridging the gap between positive and negative pressure on piston upstroke also.

However, without the addition of blowby during this vacuum period, seals and gaskets could be in an imploding type position.
That could stress the seals enough to pull outside atmosphere through the gasket / seal areas and into the engine during upstroke.
(adding more air into the system only to regenerate higher on downstroke)

If you put your finger on the end of a syringe and pull the plunger up, you get an indent in your finger from the suction on the end.
But, if you were to drill a small hole in the rubber cap on the plunger, you would not get the indent (or suction) on your finger.
The air between the rubber cap and your finger would be equalized by the air coming into that area from the outside (through the small hole).

The hole in the rubber cap on the plunger referenced above represents ring seal.
So a certain amount of blowby is needed to fill the air volume in the crankcase to just below atmospheric pressure, but not greater.
If blowby charged the crankcase with positive pressure on upstroke, the breather would open prematurely while splash and oil mist is being pulled up.
This positive air / oil charge would be amplified upon the next downstroke which normally opens the breather.
Then it darts toward the low pressure in the system which is the breather opening = oil puking out the A/C.

Ring construction
Gapless rings:
We've just historically had trouble with wet sumping when using gapless rings.
Basically, the better your ring seal, the more likely you are to have a wet sumping issue.

A lot of guys who work on a lot of gas/diesel/car/truck/bike/old/new performance stuff have moved away from the gapless second ring.
It traps pressure under the top ring.
In fact the trend has become to run the gaps looser on the second ring to allow what gets thru the top to not get stuck.

New ring / cylinder install
Neither new rings nor cylinder bores are perfectly round.
As a result, when you put a new ring on a piston and into a new bore, there's actually very little surface area of contact between the two.
The honing process puts a texture on the cylinder wall that allows the ring to machine the wall into the same shape as the ring during operation.
This increases the surface area of the contact patch.
The ring is said to be “seated” when it has carved the cylinder into it's shape and there's contact all the way around.

Many, many times I've pulled bikes apart and looked at the cylinder walls and spotted places where the ring was never touching it.
It has a huge amount to do with the accuracy of the boring job as well as the accuracy of the rings used.
If it was bored perfectly and the ring was perfectly round, there would be no seating even needed.

Ring seating during break-in
There is a risk of ring microwelding by getting too aggressive in the break-in.
HD's break-in procedure, and S&S's, and others are designed to minimize heat build-up.
Be gentle on the motor, don't put it in a situation that makes it hot.
The reason is simple.
With very little contact area between the rings and the cylinder walls;
Ring tension is concentrated and those areas that do make contact get very hot.
That localized heat can and will damage the piston, and remember, the ring land in the piston is a sealing surface.
Damage it and you'll never get a good ring seal.
This scenario happens more than you might think.
Good lubrication and a gentle break-in consisting of several heat cycles to begin with are absolutely mandatory on an Evo engine.
Ring seat depends on how good the machine work is.
If everything is perfect, it'll be seated when it's put together (it won't be though).
If the bore and/or rings are out of round badly enough they may never seat.

Normal blowby
In the absence of any blow-by getting past the rings, the crankcase alternates from atmospheric (pistons down) to a vacuum (pistons up).
But in the real world, a little gets past the rings, so there's a net outflow equal to that.

Conventional rings have a ring gap and the combustion pressure is very great.
So you can bet some of this tremendous pressure is entering into your crankcase instead of 100% of it exiting your exhaust pipes.

Excess blowby
The ringlands on the pistons 'should be' sealing but sometimes are not.
You can end up with 'out of round' or scratched cylinders from different conditions.

Testing ring seal
A leak down test is the best way to check ring seal.
The tester is not expensive and it's handy as hell.
Listen for where the air is coming out during the test: intake port, exhaust port, or breathers.

Hippysmack 1st July 2019 22:18

OK, Stop..........

It's evident that racers use vacuum pumps to increase vacuum pressure in the crankcase.
I've read that positive crankcase pressure upon piston upstroke (when the CC should be pulling vacuum) gets between the rings and basically causes bad sealing at the ringlands.
This is also in the high RPM range when ring flutter is present.
So there are several things happening then.
But racers record higher power when using a vacuum pump.
I'd like to find out more of why.

However, it has been said that inducing higher vacuum in a street engine may do more harm than good.
Lower RPM may suffer from the imbalance.

Hippysmack 1st July 2019 22:33

Engine Breathing
Engine ventilation is connected to the rockerbox, crankcase, cam gear case and the oil tank.
If you blow down the oil tank line…air comes out the rockerbox vents (or cam breather hose).
There are airways linking these compartments together and in looking at these airways.
If one pressurizes, they all pressurize and air can pass between them;

Rocker Box:
Pushrods connect the rockerbox to the gearcase.

Gravity oil:
03 and prior: Heads drain oil to the crankcase sump (through passages in the cylinders).

04 and Up: Heads drain oil to the gearcase (through passages in the cylinders and gearcase wall).

All Evos: Pushrod tubes drain oil into the gearcase.

Gearcase / cam chest:

Rockerbox oil return connects to the gearcase.
Airways thru the wall joins the sump and the scavenge side of pump.
Piston downforce pumps air and oil from the sump to the gearcase compartment.
This would equalize air pressures in the two chambers.

Piston downstroke creates a positive pressure against the oil in the sump.
This forward pressure is connected to the oil tank thru the oil pump.
It also helps to push oil into the scavenge chamber from the sump upward into the scavenger side of the oil pump.

Piston upstroke creates a negative pressure (noted as vacuum for this article).
Some of the oil either draining to the sump or collected from the sump is picked up by the vacuum in the form of oil mist.

Also some of the oil is picked up in the form of oil droplets (or splash oil) and is moved around by the next positive pressure condition.
Splash oil is further moved by the action of the flywheels, connecting rods, cam gears, air pressure and gravity.

The pinion gear shaft is hollow and connects the crankcase to the gearcase (but would only pass air with the engine off).

03 and prior engines connect rockerbox oil to the crankcase.
04 and up do not.

Oil Pump:
The pressure side of the oil pump is fed from the oil tank and is connected to the rocker box and the crankpin.

Gravity from the oil tank initially feeds the oil pump.

But once the engine starts, the motion of the gerotors creates a suction in the feed line from the tank.

Pressure in the oil tank also adds pressure on the gravity feed to the pump.
The pump creates non pressurized oil flow from the feed gerotors.

Restrictions (oil line / feed passage sizes) to the oil filter pad and through the engine create back pressure on the pump.

This pressure builds and is sent to the lifters and rocker box as well as the crankpin through the hollow pinion shaft.

The pressure is increased at the pump as oil flows through more restrictions to get to these places.
(strictly as a non tested example, 10 psi on the feed side of the pump may equate to 4 psi or lower once it reaches the crankpin)
Pressure is restricted in the cam cover,
Less restricted with the wider opening at the pinion shaft bushing,
Then restricted again thru the shaft hole and the turns in the flywheel to the crankpin.

Once the pressurized oil reaches the rocker arms and crankpin, the pressure is released into the wider openings in the oil path.
From there it is added to and becomes a part of crankcase pressure and is used and vented as such.

OEM EVO oil paths and engine breathing drawings:

86-90 engine breathing paths.

91 engine breathing paths.

92-97 engine breathing paths.

98-03 engine breathing paths.

04 and Up engine breathing paths.

Revised crankcase breathing is an area where you have huge potential to create unintended consequences.
Few if any really understand the ifs, ands and buts of all the factors the factory took into consideration when they designed the system.

Gappless rings while great for ring seal are another area where you can get in over your head if you are not careful. 41)
Wet sumping is only one potential problem you may encounter with them.
In the right application they can not be beat but you better have your ducks in a row.

Hippysmack 1st July 2019 22:42

Why Oil Pukes Out the Air Cleaner
Wet sumping is a term usually used when oil spits out the breather vent / air cleaner.
However, engine breathing and wet sumping are two different ideas.
But they are tied together as much as air and water.

The real reason these things like to spit oil out the breather is because they're a common crankpin 45° design.
Since the pistons are only 45° crankshaft degrees apart, they arrive at BDC 45° apart and again at TDC 45° apart.
So the volume of the crankcase is constantly changing.
If the check valve isn't functioning properly, it'll cause this issue.
There are usually two main concerns when this happens.

1. The breather valve(s) have to function properly.
Often the original breathers / umbrella valves will no longer work or will become less effective.
This piston design causes a variable volume crankcase:
Pistons come down and the volume is smaller, pistons go up and the volume gets larger.
Most engines don't work this way, they have a piston going up for every piston going down.
The variable volume design causes it to want to inhale and exhale air into and out of it's crankcase constantly.
For a graphic illustration of this, take your timing plug out and start your motor.

If it's allowed to suck air in, it'll have an inhalation & exhalation effect going.
Whenever air goes out, it'll carry some oil with it and deposit it.
So by allowing it to inhale & exhale, you've basically created an oil pump.

2. Likewise if the motor has excessive blowby (poor ring seal) it'll cause it.
Bad ring seal helps evacuate the sump from oil.
But on the other hand it increases the flow rate through the crank vent system to such levels that a lot of oil droplets join in.

Reasons for oil puking out the engine breather:

1. Wet Sumping:
During engine down time.
During high revs.

2. Over filling the oil tank:
If the puking starts after you top you oil tank, this is probably the problem.
If you fill the oil tank to the full mark while some oil has wet-sumped down into the engine, you have too much oil in the system.
The oil from the sump will be pumped back up to the tank, dribble down the vent tube to the timing cover, from where it is fired out the engine breather.
This puking will continue after initial start up until all the excess oil has been fired out, which can take a while.
The cure is to drain a quart or so out of the oil tank, run the engine for five minutes until the puking stops, then top up the oil tank to the full mark.

3. Worn engine / rings:
If your engine breather continues to puke oil or blow smoke after the above two things have been eliminated, your problem is most likely wear in the cylinders and heads.
Worn rings and even valve guides, can allow blowby of combustion gasses into the crankcase area, which then comes out the breather.
Usually this will be accompanied by smoke or oil coming out the exhaust pipes too.
A compression test will give some indication of top-end condition.
Anything below 120psi is suspect, according to the factory manual.

These bikes will still run OK at even 100psi, but they will be down on power and consume oil, and blow fog out the breather pipe.

The OEM system is spec'd with a certain pressure on downstroke in mind.
If atmospheric pressure is also present then, the total positive pressure will be higher at the breather vent when it opens next.
The extra air is combined with what the downstroke exerts (including normal blowby).
Likewise, increased blowby adds more air in the crankcase which increases positive pressure.
The higher the positive pressure is when vented, the greater the oil that is carried out with it.
It's a balance.
There are very small amounts of oil mist that will normally go back into the carb.
You may not see it, but it is there.
When the balance is off, you see it though.
When we modify the system, these balances have to be maintained.
That may be in the form of a new or better breather valve.
Or that may require a better breather valve in addition to something else.

Worn rings / leaking oil out the umbrellas is only one cause of oil loss, however;
The one way breather valve is designed to only allow air out.
This helps restore the OEM aim of a crankcase vacuum.
It also cuts down on oil leaks.

Once the umbrella valve(s) fail, it changes to an open breathing system that allows air to suck in and blow out.
Violent air pressure changes and power losses follow.

The fluctuations in air pressure suck oil up into the air in the engine.
The fluctuations are driven by the pistons.

So to keep oil out of the A/C, means keeping engine breathing And wet sumping in balance.

Hippysmack 1st July 2019 22:53

Breather Valves
76< engines have rotary breather valve on the oil pump that is timed to open on the downstroke of the pistons.
This allows crankcase exhaust air pressure to expel the scavenge oil from the crankcase breather oil trap into the gearcase.
The breather valve closes on piston upstroke, creating vacuum in the crankcase.

77> engines do not have a breather gear on the oil pump.
Without the breather gear that means you have no control of the oil/air density of the crankcase.

1957-1976 models:
There is a timed breather valve built into the oil pump drive, which vents crankcase pressure into the cam timing chest.
A six-inch metal tube hanging down from the timing cover near the generator drive, at the 6 o’clock position vents that controlled pressure to atmosphere.
A metal disc on the end of the generator drive gear centrifugally separates oil from the air as it is discharged overboard.
1977-78 models only:
The timed breather on the oil pump drive was dropped.
An external non-return valve was plumbed into that vent tube sticking down from the timing cover at the generator drive.
This allows air out, but not in. It is sometimes referred to as the foo-foo valve.

1979-Early 1982 models:
The external foo-foo valve and the six-inch metal vent tube at the front of the timing cover were done away with.
Instead, a one-way foo-foo valve was built inside the timing cover.
(reed valve assembly 26909-79A) A rubber breather hose then ran from the generator drive area of the timing cover, at the 9 o’clock position.
It connected to the stock air filter so that any oil mist was fed back through the engine.
This made the EPA more happier than they were with the idea of engine oil spraying out into the atmosphere.
Many of these bikes with custom air filters simply run that hose down to the bottom of the frame.
(and let the oil mist blow out in the time honored manner)

Late 1982-Early 1984 models:
The internal crankcase breather valve was redesigned to incorporate a rubber umbrella valve attached to the base plate along with a larger diameter (1-3/4” O.D.) oil separator washer on the generator armature.

Late 1984-1990 models:
A breather baffle tube system was incorporated into the cam cover in the area behind the oil filter.
The baffle tube has a one way umbrella valve mounted into it.
There is a plug in the 6 o-clock position and a tube in the 11 o-clock position going to the air cleaner.
On each piston downstroke, crankcase pressure (air and oil mist) is routed to the breather baffle at the front of the gearcase.
Oil is separated from the air pressure by the one-way umbrella valve.
The oil then drains into the gearcase through a drain hole in the breather valve.
Exhaust air escapes past the one way umbrella valve in the baffle tube and into an outlet fitting on the cam cover.
The air is then routed into the rear of the air cleaner via an oil hose to the gearcase outlet.

1991-2003 models:
In 1991, along with the 5-speed transmission, the MoCo moved the crankcase breathing from the cam box cover outlet to breather bolts in the heads.
Previously, these bolts were just used to mount the carb and air cleaner to the heads.
The new breather system uses one-way umbrella valves in the rocker boxes.
These exit crankcase vapors through vents in the top of the cylinder head into the carb mouth to be burnt. 57)
On each piston downstroke, crankcase pressure (air and oil mist) is routed up the pushrod tubes into the rocker box.
Collected air pressure and oil mist in each rocker box is routed up into a sealed cavity in the lower portion of the box.
This mixture passes up from underneath a rubber one-way valve (umbrella valve) sitting over the cavity inlet.
The oil is designed to separate from the air by hitting the underside of the umbrella valve.
Then dropping back down into a recessed area behind the umbrella valve in the cavity.
From there it should drain back into the main rocker box through a tiny hole behind the umbrella valve and then back to the lower end.
Air pressure is designed to continue up past the umbrella valve and exit a hole in each head on the intake valve side.
Air pressure escapes the head through the hollow bolts (one in each head) that hold the air cleaner mount.

The 91-03 umbrella is sitting on the middle box spacer inside an enclosed cavity when it's buttoned up.
But this cavity has a tiny oil drainback hole on the left side of the umbrella.
So, if you blow into one head vent (carb bracket off and umbrella in good shape), the air comes out the other head vent.
There is the restriction of the tiny hole you're blowing air into.
So you will have to pucker up to blow hard through it.
That being said, that tiny hole will allow a small amount of outside air back into the crankcase on piston upstroke.
As soon as the sealed cavity is drained from separated oil, the only thing left is outside air to pull into it.
(for a small amount of time until the piston falls again)
This, in affect, will add a slight atmosheric aide to the 'then' negative crankcase pressure.

Just as you can't pump into a container with no vent, you can't suck from a container with no back vent.
Gravity drain oil (heads / pushrod tubes) also has the force of suction to help pull it down on piston upstroke.
Likewise, that suction also helps to drain the tiny hole in the sealed rocker box cavity.
So the head vent also doubles as a back vent to help CC vacuum drain the sealed cavity on piston upstroke.

In 2004 the MoCo made some changes to the umbrella valve configuration.
The umbrella was retained but now inside a plastic housing with a pre-umbrella oil separating screen.
Each 'breather valve' is assembled into a plastic fitting that is sealed over the outlet hole in the lower rocker box to the head breather bolt.
The new breather valves were also accompanied with new style hollow air cleaner mounting bolts for the pressure to escape.
The new bolts are the same thread size as previous.
But instead of a simple hex, it also has a shoulder past the hex for an O-ring to be fitted between the hex head and the air cleaner.
The breathing system is functionally the same as 91-03 with the one-way umbrella valves in the rocker boxes.
These exit crankcase vapors through vents in the top of the cylinder head and into the carb mouth to be burnt. 58) On each piston downstroke, crankcase pressure (air and oil mist) is routed up the pushrod tubes into the rocker box.
Collected air pressure and oil mist in each rocker box is routed up into the breather valve unit in the lower portion of the box.
This mixture passes up from underneath the breather unit.
The oil is designed to separate from the air by hitting the underside of the screen / umbrella valve and dropping back down into the rocker box.
From there it is routed back to the lower end.
Air pressure is designed to continue up past the breather unit and exit a hole in each head on the intake valve side.
Air pressure escapes the head through the hollow bolts (one in each head) that hold the air cleaner mount.

The 04 Up breather does Not have the backvent mentioned above as the 91-03 setup does.
So when the umbrella closes, atmosphere should be locked out from entering the engine.
Implications of whether a that's good or bad thing I'm unclear of.

To keep the engine from pulling in air from the outside, HD fits the breather vent(s) with a check valve.
This way, the motor can exhale, but not inhale.
The reason for the check valve is not to keep it from pulling in dirt.
Any inhaled air is going to get pushed out again and take oil with it.

We have check valves in the rocker boxes (or cam chest respectively) that are *supposed* to eliminate this.
If they aren't functioning properly, you'll get lots of oil out the breathers.

The valves themselves rarely fail (unless they get hard), but the casting they sit in is, well, a casting…not very accurate.

Buell had a service bulletin years ago that talked about chamfering the umbrella valve hole to make it seal better.
(as a fix for excess oil in the air cleaner)

The breather valve not only has to work but it has to operate in sync with the action of the pistons.
It has to open with downstrokes and close (completely) with upstrokes.

All times are GMT +1. The time now is 01:13.

Powered by vBulletin® Version 3.8.11
Copyright ©2000 - 2023, vBulletin Solutions Inc.
XL Forum® - Linson Media LLC